Olefin polymerization catalyst systems can be classified into Ziegler-Natta and metallocene catalyst systems. Among them, the metallocence catalyst comprises a combination of a main catalyst whose main component is a transition metal compound, and an organometallic compound cocatalyst whose main component is aluminum. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and a uniform composition distribution of comonomers, depending on the single site characteristics. The metallocence catalyst has characteristics which can control the stereoregularity, copolymerizing properties, molecular weight, crystallinity degree and the like of the resulting polymer by changing the ligand structure of the catalyst and the polymerization condition.
On the other hand, a linear low density polyethylene (LLDPE) is produced by copolymerizing ethylene and alpha olefin using a polymerization catalyst under low pressure. Thus, this is a resin having a narrow molecular weight distribution and a certain length of a short chain branch, without a long chain branch. The LLDPE film has high strength and elongation at break in addition to the properties of a general polyethylene and exhibits excellent tear strength, falling weight impact strength or the like. This has led to an increase in the use of a stretch film, an overlapping film or the like which is difficult to apply existing low density polyethylene or high density polyethylene.
However, LLDPE has poor processability for a blown film compared to excellent mechanical properties. A blown film is a film produced by a method of blowing air into a molten plastic and inflating it, which is also called an inflation film.
As factors to be taken into consideration when processing a blown film, bubble stability, processing load, etc. must be considered, and especially, bubble stability should be considered important. The bubble stability means a property that, when the film is produced by injecting air into the molten plastic, the produced film maintains its shape without being tom, which is associated with a melt strength (MS).
The melt strength refers to a strength for maintaining a shape capable of withstanding the molding and processing in a softened and melted state. The melt strength of low density polyethylene (LDPE) is higher than that of LLDPE. This is because in the case of LDPE, branched chains are entangled with each other as compared to LLDPE, which is more advantageous in withstanding the molding and processing. Therefore, in order to complement the melt strength of LLDPE, a film is produced by blending LDPE, but there is a disadvantage that it is impossible to avoid degradation of physical properties due to blending of LDPE.
In view of the above, the present inventor has found that LCB (Long Chain Branch) is introduced into LLDPE and thus, the low density polyethylene having a remarkably high melt strength can be produced without blending with LDPE. The present invention has been completed on the basis of such finding.